A known developer cartridge includes a casing and a baglike developer container disposed in the casing. The developer container contains developer. The developer container has an outlet through which the developer flows out of the developer container. The outlet is sealed by a film-like sealing member. The sealing member is fixed to a shaft of an agitator. Rotating the agitator causes the sealing member to be separated from the developer container. This allows the developer to be discharged from the developer container to the casing of the developer cartridge through the outlet.